In waterflooding projects, the techniques of water diversion employed depend on the degree of heterogeneity of the porous media or strata being treated. The more permeable zones of the subterranean formation tend to take most of the injected fluids. While initially this is acceptable in sweeping oil from such zones of relatively high permeability, it subsequently becomes undesirable as the oil content of such strata becomes depleted since much of subsequently injected flood water or other fluid by-passes the relatively less permeable zones and provides little benefit in enhancing further hydrocarbon recovery.
An isolated high-permeability zone or fracture can be plugged at the well core face by a shallow layer of applied cement, though such a permanent relatively irrevocable technique often is undesirable. More desirably, a communicating high-permeability zone preferably is plugged to some considerable depth in order to be most effective in preventing flood water from otherwise merely flowing around a narrow shallow plug and back into the high-permeability or swept zone. In depth plugging of a relatively high-permeability zone converts the zone into a much lower permeability zone. Then, subsequently injected flood water or other fluid will tend to enter formerly by-passed but now relatively more-permeable hydrocarbon-bearing zones and thus mobilize increased amounts of hydrocarbons therefrom.
In depth plugging can be effected by the injection of gelable polymer-thickened aqueous solutions containing sequestered polyvalent metal cations which cause the gelation or crosslinking of the polymer-thickened solutions when the pH of the solution is in a gelation range. Alternatively, gelation can be effected by a redox system in which a polyvalent metal in a reducible oxidation state is used in combination with a reducing agent.
These gel-plugging techniques also are used during well workovers, in fracture treatments, and to correct the injection profile of a well by in depth sealing of streaks of relatively high permeability so that flooding fluids will enter the formation in a more useful front profile.
In many operations it is desirable to plug a portion of an underground formation so as to divert subsequently injected treating fluids into certain zones of the formation and yet be able to remove the stoppages from the formation after the particular treatment is completed.
An advantage in using gel-forming compositions for controlling the permeability of underground formations during waterflooding and chemical flooding procedures is the controllable nature of such blockages, since gel-breaking can be accomplished, when desired, by appropriate subsequent treatment.
Various methods have been used in the past to achieve in depth gelling, such as gelable systems triggered by a following aqueous acidic solution injection for subsequent pH adjustment. However, injecting an acidic solution following the polymer solution may result in gelation occurring so rapidly that a sufficient in depth plugging is not effectively obtained in the most permeable strata where desired. In another method, water, a polymer and a crosslinking agent capable of gelling the polymer such as a sequestered polyvalent metal cation, are admixed, and, just before injection into an underground formation, an acid is added thereto to effect gelation. But, when the acid is pre-mixed with the gelable composition, the gelation can be too fast, resulting in the necessity of shearing the gelled polymer in order to be able to obtain adequate injection, which reduces effectiveness of the gel.
Needed is a gelable injectable liquid composition which has an injection pH outside the gelable pH range yet processes the capability of forming the desirable gel in situ, without the need for additional follow-up injections of acids or other gel-triggering reagents, and method of employing same.